The present invention relates to a process and system for the abatement of impurities being contained in gas stream, in particular a gas stream being rich in hydrogen and carbon monoxide.
Hydrogen and carbon monoxide rich gas is commonly known as synthesis gas. Synthesis gas is a useful feed stock for the preparation of chemicals, including methanol, dimethyl ether, ammonia or the Fischer-Tropsch synthesis.
The preparation of synthesis gas from several sources like natural gas and naphtha by steam reforming is well known in the art. Lately, synthesis gas preparation by means of gasification of solid and liquid fuels, such as coal, pet-coke, biomass, and waste of various character, has attained increasing interest because of decreasing availability of natural gas and liquid hydrocarbon sources.
A problem with the use of synthesis gas prepared by gasification is the relatively high content of impurities, which have a poisonous effect on particular catalysts being employed in the subsequent conversion of the gas to chemicals.
The poisonous contaminants comprise mainly sulphurous compounds, particularly carbonyl sulphide, metal carbonyls, carbon disulphide and hydrogen sulphide together with hydrogen cyanide, ammonia and arsenic and chlorine compounds. These compounds poison catalysts with e.g. copper or zinc or zeolites as the catalytic active material in an irreversible manner by forming metal sulphides, cyanides, arsenides and chlorines.
To avoid serious poisoning of downstream catalysts the content of the contaminants in the synthesis gas has to be substantially reduced to the low ppb range, preferably below 10 ppb.
Bulk amounts of hydrogen sulphide in the synthesis gas are in industrial processes conventionally reduced down to the ppm range by washing processes with chemical or physical solvents, comprising the known Selexol and Rectisol process using organic compounds as physical solvents or amine wash processes employing alkanol amines as chemical solvents, like the monoethanol amine (MEA) and the methyldiethanol amine (MDEA) process.
Reduction of trace amounts of sulphurous compounds from gas streams is furthermore known in the art.
EP 320979A2 discloses desulphurization of a hydrocarbon stream to a sulphur content below 5 ppb by employing a copper-zinc desulphurizing agent.
Use of chromium, iron, cobalt, copper, cadmium, mercury or zinc alumina spinels as sorbent for the removal of hydrogen sulphide is known from U.S. Pat. No. 4,263,020.
Removal of carbonyl sulphide, hydrogen sulphide and hydrogen cyanide from a synthesis gas stream to less than 2 ppb in single guard bed with iron-oxide based material is discussed in WO 2007/093225. This document further discloses reduction of amounts of hydrogen cyanide, hydrogen sulphide and ammonia either by a simultaneously or serially treatment with zinc oxide.
The prior art fails to teach entire removal of all impurities in a feed gas stream being a catalyst poison in a subsequent catalytic process for the conversion of the feed gas into chemicals.